Mechanisms controlling the phenotypic expression of Genetic Resistance to a Flaviviral Encephalitis are being studied in 2 strains of C3H mice one of which is resistant (C3H/RV) and the other of which is susceptible (C3H/He) to lethal infection. Recent results indicate that immune factors as well as production of defective interfering virus contribute to survival of resistant mice. Therefore the primary immune responses of the 2 strains of mice will be compared in vivo and in vitro. The kinetics of defective interfering virus production will also be evaluated. Finally, the hypothesis that qualitative and/or quantitative differences may exist between the 2 strains in cell membrane virus receptors on target cells in the brain and lymphoid tissues will be examined.